The present invention relates generally to turbine flow meters, such as those shown and described in U.S. Pat. Nos. 6,854,342, 6,692,535 and 5,689,071 (each of which is incorporated herein by reference in its entirety), for use in fuel dispensing environments. More particularly, the invention relates to a turbine flow meter adapted to have enhanced accuracy during low flow rate conditions.
Turbine flow meters may be used in a variety of applications in fuel dispensing environments. For example, turbine flow meters may be used to meter fuel being dispensed, measure the vapor being returned to the underground storage tank in a stage two vapor recovery system, or measure the vapor or air released to atmosphere from the ullage area of an underground storage tank when a pressure relief valve in a vent stack is opened to relieve pressure.
Turbine flow meters generally comprise a-housing having inlet and outlet ports at respective ends thereof. A shaft is located inside the housing along the housing's longitudinal axis. A turbine rotor mounted on the shaft rotates when fluid (liquid or gas) flows through the housing via the inlet and outlet ports. The rotor is made of a magnetic material such that its rotation is detected by a pickup coil mounted to the housing. As a result, the flow rate of the fluid flowing through the housing can be determined.
In some cases, the meter may have two turbine rotors, one located upstream of the other. If a respective pickup coil is provided for each rotor, the rotor frequency of each rotor can be determined. A controller divides the second rotor frequency by the first rotor frequency to derive a frequency ratio. This ratio can be used to determine the flow rate of the fluid flowing through the meter.
In a two-rotor meter, the downstream rotor will usually rotate even under low flow rate conditions. However, the velocity of the fluid may not be sufficient at low flow rates to rotate the upstream rotor. As a result, it may not be possible at low flow rates to determine the frequency ratio of the two rotors (and thus the fluid flow rate).
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.